C. It is often the case. At least for lightly engineered, simple circuits.

B. The common implementation is a series inductor between VCC (frequently at 5V) and the AVCC pin. Then all of the internal analog circuitry is slightly isolated from the "noisier" digital corcuitry.

A. VREF tied to 5V (for example) is the voltage that the analog system compares the analog input to. Tying to 5V is reasonably simple and convenient. Imagine this, though: If you had a device that provided 2.560 volts as a reference to an 8-bit analog input, then each bit would represent .010 volts. A 1.76 volt input would read as "176" when you read the value of that analog port. Nice, right? (If it were tied to 0V, then all analog readings would be "0".)